The 3G architecture also has the concept of security contexts as in LTE, and this becomes very important when 3G and 4G devices and network entities inter-operate. A 3G device that was initially attached to a 3G network has received a set of security content that is stored in the UICC. This information is considered a partial and legacy security context in the 4G environment. For a security context to be considered full, the MME should have the K-NASenc and K-NASint keys, which are obtained when the 3G device handovers to the 4G network. In this handover case, the legacy security context is called mapped EPS security context.
